UNDERSTANDING WHEN TO SELECT 2-D OR 3-D SEISMIC

Based upon a workshop co-sponsored by PTTC's South Midcontinent Region on July 29, 1999, in Oklahoma City, OK.

BOTTOM LINE

Three-dimensional (3-D) seismic data can improve the probability of a successful drilling program, but it adds significantly to a project’s cost. Thus, the value and limitations of 2-D versus 3-D data must be clearly understood to determine when/ if 3-D data are required.

PROBLEM ADDRESSED

Because of the cost involved, operators must have a clear idea of the value of a seismic survey. For instance, 3-D seismic surveys can provide data to map the subsurface more accurately than with 2-D data. Usually 3-D surveys are economically successful for smaller survey areas where the structure and stratigraphy is complex enough to justify the additional cost. When selecting a 3-D seismic survey, it is important to consider cost, time required for acquisition and processing, and access to computer workstations, among other factors.

KEY WORDS:

Seismic Survey, 3-D Seismic, Advanced Seismic, Geophysics

SPEAKERS

Introduction to Geophysical Theory; 2-D or 3-D Seismic;
Welder Ranch Survey, Texas; Workstations
Deborah K. Sacrey, Auburn Energy

Identifying Reflectors; Purchasing Seismic
Ray Brown, Oklahoma Geological Survey

3-D Examples, Southern Oklahoma
Bob Springman, independent geophysicist

Chester Formation in South Eubanks Field
Ernie R. Morrison, Kansas Geological Survey

TECHNOLOGY OVERVIEW

A 3-D seismic survey is basically a dense grid of 2-D lines. However, the way in which it is processed and interpreted is different from that of 2-D data. This difference enables practitioners to take the data from linear (2-D) to volume (3-D) information. In 3-D surveys, the subsurface is closely sampled in every direction, leaving no open loops or gaps in the structure or stratigraphy when it is analyzed.

The greatest advantage of 3-D data is that it provides the ability to map the subsurface more accurately, allowing the interpreter to fill in the “gaps” that develop when using only 2-D data. Other advantages of 3-D over 2-D include: a greater density of data points, more accurate positioning, improved horizontal and vertical resolution, and a choice of viewing perspective.

There are several other considerations for 3-D data:

• Cost. 3-D data are much more expensive to acquire and process than 2-D lines. The average cost for a 3-D survey in the Midcontinent region is $30,000 per square mile.
• Time. 2-D data require much less permitting, surveying, and processing time than even small 3-D surveys. Large 3D seismic shoots may take 1 to 2 years to acquire, and 3 to 4 months to process.
• Need. 3-D surveys are appropriate for those looking for complex structures or subtle stratigraphic changes to delineate potential traps.
• Use. 3-D data interpretation is better accomplished using computer workstations rather than printouts. However, this requires access to an interpreter and workstation.

In deciding whether 3-D data are needed, the following questions should be considered: 1) Is the structure or stratigraphy sufficiently complex to require 3-D data? 2) Will a 2-D grid miss overlooked reserves? 3) Do current geological and engineering data conflict? 4) Are secondary/ tertiary recovery attempts indicating barriers?

Advanced seismic data techniques are being used in some regions. For instance, multi-component data, using a combination of compressional and shear data, has been successfully applied, primarily to carbonates in the Rocky Mountains and the North Sea. This process is almost twice as expensive as normal 3-D acquisition. Using variations in seismic response over time (4-D seismic data) is useful to map subtle changes such as depletion or remaining attic reserves. Because of their expense, 4-D surveys are usually conducted over large prolific fields as part of an ongoing reservoir characterization.

LESSONS LEARNED

The cost of a modern 3-D seismic survey can become a significant part of a well’s cost. Risk analysis can be one means to help operators decide if 3-D techniques should be used. This involves various methods of comparing the value of information to expected monetary value that has been weighted with economic factors.

FIELD RESULTS

Welder Ranch Field 3-D Seismic
Welder Ranch field (south Texas) has been heavily explored since the late 1930s. Natural gas, condensate, and oil production in the area are primarily from the Frio Formation, which is more than 4,000 ft thick in the area. A 90-square-mile 3-D survey was conducted in 1997. More than 1,200 well logs were correlated and a digital base of well information was purchased. The density of wells within parts of the study area indicated a high risk for new exploration wells. Structure maps made before the seismic survey were compared to the 3-D maps. The 3-D seismic data, combined with coherence mapping, showed that faulting and other structural aspects of the middle Frio were much more complex than previously believed and that separate fault blocks were present downdip to production. Structurally high positions within newly recognized fault blocks were compared to well and production data. After drilling, significant reserves were discovered that would not have been located without the 3-D seismic survey.

Seismic Data From Oklahoma
The structure and stratigraphy of most of Oklahoma can be addressed using seismic data, correlated with available well control. For structural exploration, operators should identify a reflector showing the structure of the target horizon. For stratigraphic mapping, the target horizon must be identified and mapped directly. Once correlations have been made, it is easy to map structural and stratigraphic targets such as the Cromwell or Booch channel sands. Examples from Garvin County indicate that 3-D seismic reduced risk and identified new prospects. In the study area, 2-D seismic methods were capable of mapping reflectors; however, they did not offer the density of coverage necessary to find additional reserves. Examples from the Whitebead Oil field, South Brady field, and North Brady field illustrate that 3-D seismic surveys led to drilling additional productive wells. Discoveries of the basal Oil Creek sand also led to detecting additional zones at other horizons.

Chester Formation In South Eubanks Field
More than 40 wells of Mississippian Chester reserves in the South Eubanks field (Haskell County, Kansas) were studied in the Hugoton embayment of the Anadarko Basin within the boundaries of the Permian Hugoton gas field. Early deep drilling was based on extrapolating shallow subsurface data to predict deeper structures. A 2-D seismic survey in the 1980s worked well for imaging major structures. However, it did not depict smaller structural features or show where the Chester sandstone was deposited on the underlying Ste. Genevive or St. Louis Limestone. A 26-square-mile 3-D survey was shot and a narrow (less than 1,000 ft) channel was imaged. Fourteen successful wells were drilled in the major scour. The 3-D survey also indicated a series of karst features in the Chester and deeper strata that have not yet been tested.

CONNECTIONS:

Deborah K. Sacrey,
Auburn Energy The Ashford Atrium,
14825 St. Mary's Lane, Suite 118
Houston, TX 77079
Phone 281-493-5059, fax 281-493-3390, E-mail auburn@concentric.net

Ray Brown,
Oklahoma Geological Survey
100 East Boyd St.,
Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-366-2882, E-mail raybrown@ou.edu

Bob Springman, independent geophysicist
820 Glenlake Drive
Edmond, OK 73013
Phone 405-755-5085, Fax 405-775-0808, E-mail rspringman@aol.com

Ernie R. Morrison,
Kansas Geological Survey
107 North Market, Suite 408
Wichita, KS 67202
Phone 316-265-7614, Fax 316-788-3863, E-mail jerm@fn.net

For information on PTTC’s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, E-mail cjmankin@ou.edu

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